im an overall friendly person so i can see from her point of view. obviously your ex girl has moved on and can feel comfortable being around mutual friend and your current interest.. no big deal with that.
i mean i would like to hope that was the reason, maybe it is some sly sabatage shit. but ive stayed friends with ex's and been comfortable around their new girl with no problems.. and if the gf isnt cool around me..than whatever and if shes cool with it..than fine